If you upgrade to a quad core (preferably an intel i5 or i7, AMD cpus tend to bottleneck more than intel ones) 3.5GHz and get a decent GPU (around a geforce 650 or higher) you will have much better luck reaching full speed on many more games than you will now. I dont know how demanding most of those games are, but if you were to get at least the above specs you would probably be fine with the majority. Upgrading to at least 8gb of RAM wouldnt hurt but is unecessary for pcsx2.

It depends on the game. You'll definitely have to upgrade your rig to get fullspeed on most of those games, regardless of how much you overclock that CPU or use speedhacks. If you want info on what games will be able to run at fullspeed, use the search function on this forum to search for a game and see if others have talked about it. Otherwise you can check YouTube and get a rough idea of how easy/hard the game is to run. There's no such thing as a universal setting or performance rating for all the games on PCSX2...